The present invention relates to a process for preparing composite fibers and diaphragms as used, for example, in chlor-alkali electrolysis.
There are basically two types of chlor-alkali electrolytic cells for the production of caustic soda and chlorine from sodium chloride: mercury and diaphragm. In the diaphragm process, a porous diaphragm separates the anode and cathode compartment. An aqueous sodium chloride solution flows from the anode compartment through the diaphragm into the cathode compartment, where hydrogen is produced at a steel cathode. The effluent cell liquor comprises sodium hydroxide as well as sodium chloride. The chlorine produced at the anode is obtained in gaseous form. Modern diaphragm cells feature adjustable, activated titanium anodes and increasingly diaphragms densified with synthetic polymer fibers instead of the traditional asbestos diaphragms.
Diaphragms are formed of a basic structure of organic polymer fibers which holds inorganic materials. Various processes for preparing such diaphragms or for preparing the composite materials used for preparing the diaphragms are known.
U.S. Pat. No. 4,680,101 describes a process for preparing diaphragms by mixing a dispersion of polytetrafluoroethylene (PTFE) fibrils, polypropylene fibers and a perfluorinated ion exchange material in water and applying the slurry to a perforated steel plate cathode covered with a cellulose filter paper. After removal of the volatiles, the diaphragm is dried at from 120xc2x0 C. to 130xc2x0 C. and, after cooling, impregnated with a solution of partially hydrolyzed silicon alkoxide and zirconium alkoxide. Then the diaphragm is dried again.
EP-B-0 196 317 describes a process for preparing fiber composite materials by using a ball mill to hot mix a PTFE dispersion with zirconium dioxide and sodium chloride, which initially causes the dispersion medium to escape. After the mixing, the product obtained is separated from the ball media used. It comprises irregularly shaped, partly branched fibers consisting of a composite of the PTFE used and the finely divided zirconium dioxide. The second inorganic material, sodium chloride, assists in the fiber formation process and can be dissolved out by the brine before or during the subsequent application. The fibers obtained can then be used to prepare a diaphragm. Prior art diaphragms do not always exhibit the desired high flow resistance, which prevents backmixing of the caustic obtained during the electrolysis. The diaphragms obtained are accordingly not of sufficient quality for all applications.
Not all the above-described process variants are suitable for preparing fibers for chlor-alkali electrolysis diaphragms. Not just any branched fiber can be used for preparing chlor-alkali electrolysis diaphragms. The diaphragms obtained from the fiber do not always have the required defined flow resistance.
The flow resistance of a diaphragm determines the rate of flow of the brine through the diaphragm. The flow rate also depends on the pressure forcing the brine through the diaphragm. In the field, the pressure is regulated by the difference in head between the brine feed and the catholyte effluent. Suitable values range, for example, from 20 to 70 cm of liquid column. This flow rate in turn has a direct bearing on the concentration of the caustic produced. In addition, the applied current density has no influence on the optimal flow rate. The concentration of caustic obtained should range from 100 to 150 g/L. In the field, this requires flow rates of 20-30 L/m2h and current densities from 2 to 2.5 kA/m2, for example.
The use of a ball mill for preparing the fibers leads to problems due to incomplete removal of the water in the dispersion. Said incomplete removal of water can cause rusting of the steel balls used, in which case PTFE will collect on the rust-roughened surfaces of the steel balls, preventing adequate fiber formation. To circumvent this problem, the starting materials have to be mixed and dried in another apparatus. This makes the process costly. In addition, at the end of the ball milling step, the balls used have to be separated off to isolate the fibers. This separation step is costly. It can take the form of sieving, for example.
It is an object of the present invention to provide a process for preparing such composite fibers as permit the preparation of diaphragms having a defined flow resistance to meet the technical requirements of a chlor-alkali electrolysis cell.
We have found that this object is achieved according to the invention by the process for preparing composite fibers by
(a) mixing a PTFE or PTFE copolymer dispersion or powder with a finely divided inorganic material and a fiber forming material,
(b) shear heating the resulting mixture to a temperature at which sheared PTFE or PTFE copolymer becomes flowable without showing signs of de-composition while removing the dispersion medium, if a PTFE or PTFE copolymer dispersion is used,
(c) cooling the mixture to below 70xc2x0 C.,
(d) mix shearing the mixture at below 70xc2x0 C. to form the composite fibers.
The invention proposes that shearing the mixture of PTFE or PTFE copolymer, finely divided inorganic material and fiber forming material especially at less than 70xc2x0 C. provides fibers which permit the preparation of improved diaphragms having a defined flow resistance.
The heating in step (b) is preferably to more than 70xc2x0 C., particularly preferably to more than 100xc2x0 C., especially 130-180xc2x0 C. Thereby coarse clumpy fiber hanks are formed. The cooling in step (c) and the shearing in step (d) are each preferably carried out at 20-60xc2x0 C. A lower temperature in step (d) makes the mixing and shearing more difficult because of the increased stiffness of the material. In this step a chopping of the material and a separation into free flowing fibers is performed.
The invention further proposes that the shearing of the mixture in step (d) is advantageously carried out in mixers at a Froude number of more than 1. This requires the use in this step of mixers having a Froude number of more than 1. In this case the cooling in steps (c) and (d), respectively, is not necessary.
The Froude number is a measure of the intensity of mixing and is defined as Fr=r2/g where =2 .f, f=fre-quency, r=radius, g=gravitational constant. The frequency is determined from the speed of the mixing tool. The radius is the largest distance between the mixing tool and the shaft.
Examples of suitable mixers are Eirich mixers, ring tub mixers, ring layer mixers, DRAIS mixers. It is similarly possible to use a Lxc3x6odige mixer fitted with additional choppers whereby Froude numbers of more than 1 can be achieved. A particularly preferred high intensity mixer is an Eirich mixer which is characterized in that it has a rotating mixing pan and a mixing tool rotor which selectively rotates or contrarotates. The mixing tool can reach a very high speed of more than 2000 rpm. The mixing tools are whisk- or stirrerlike tools which can have diverse geometric shapes and which ensure thorough mixing and an input of a high level of mixing energy. A wall scraper prevents material sticking to the walls. Eirich high intensity mixers are available from Maschinenfabrik Gustav Eirich, Hardheim, Germany.
The process can preferably be carried out in a vacuum mixer which can be heated. Vacuum mixers are provided by Eirich. These mixers perform the so-called EVACTHERM(copyright) process (of Eirich).
The heating of these mixers is performed by steam or hot steam which is led directly onto the mixture, and by the heating jacket of the mixer. The temperature of the jacket which is heated with steam as well may be adjusted by applying pressure or lower pressure. This specific advantage of these mixers is the possibility to rapidly cool the content. By injecting water and subsequently evacuating the mixer content may be cooled to the desired temperature (less than 70xc2x0 C.). The invention relates also to the use of these types of mixers with a Froude number of more than 1 in the production of composite fibers.
Customary mixers, such as Brabender mixers, Banbury mixers and Houbart mixers or ball mills, cannot attain a Froude number of more than 1. Ball mills, in particular, additionally have the disadvantages mentioned in the introduction.
The process of the invention provides fibers which are dry and free flowing. This is achieved especially by using the high intensity mixer in step (d). The aforementioned high intensity mixers are also used with particular preference in step (b) of the process of the invention. More particularly, all steps of the process of the invention are carried out in one and the same high intensity mixer, so that there is no need for any transfer during the process. The resulting fibers, which are dry and free flowing, are simple to remove from the mixer. In contrast to ball mills, moreover, the costly removal of the balls from the fibers is obviated. The multistep nature of the process, especially drying and fiber formation at high temperatures and fiber comminution at lower temperatures, permits control of the properties of the fibers in a specific manner, making it possible to set the flow resistance of the diaphragms prepared therefrom.
The PTFE or PTFE copolymer dispersion used in step (a) is preferably an aqueous dispersion. Following mixing with the finely divided inorganic material and the fiber forming material, step (b) comprises removing the dispersion medium, preferably water, by heating and commencing fiber formation by shearing. Following the cooling of the mixture in step (c), step (d) comprises finishing the fibers by comminution to obtain the free flowing fiber material of the invention.
The fiber forming material used is preferably an alkali metal salt or an alkaline earth metal salt. It is preferably an alkali metal halide or an alkaline earth metal halide. Particular preference is given to sodium chloride, magnesium chloride, calcium chloride or else sodium carbonate, with sodium chloride being used in particular. The particle size is preferably less than 300 xcexcm, more preferably less than 200 xcexcm, particularly preferably less than 100 xcexcm, for 90% by weight of the particles. A typical preferred particle size distribution is as follows: 10% less than 5 xcexcm, 50% less than 40 xcexcm, 90% less than 80 xcexcm.
The finely divided inorganic material used can be an inorganic material which is chemically stable under the conditions of chlor-alkali electrolysis. It must be stable to strong alkalis, acids and oxidizing media, such as chlorine. Finely divided inorganic material used is preferably an oxide, carbide, boride, silicide, sulfide, nitride or silicate such as ZrSiO4 or an alumosilicate or aluminate, except asbestos, especially a transition metal oxide. The material should be stable in acidic and alkaline aqueous media. The use of zirconium oxide is particularly preferred. The average particle size of the finely divided inorganic material is preferably less than 100 xcexcm, particularly preferably less than 40 xcexcm, especially less than 10 xcexcm. A preferred particle size distribution is as follows:
10% less than 0.5 xcexcm
50% less than 1.2 xcexcm
90% less than 5.7 xcexcm
A further preferred distribution is as follows:
10% less than 0.63 xcexcm
50% less than 1.74 xcexcm
90% less than 10.18 xcexcm
The PTFE or PTFE copolymer dispersion is prepared by dispersing PTFE or PTFE copolymer, preferably in water, in the presence of a dispersant, especially of a nonionic surfactant in an amount of 1-10% by weight, based on the PTFE or PTFE copolymer.
Preferred dispersions are prepared by emulsion polymerization. The solids content is preferably from 30 to 80%, particularly preferably from 50 to 70%. The viscosity of the dispersion is preferably from 7 to 13 mPas at a shear rate of 4000/s. The particle size is preferably within the range from 100 to 500 nm, particularly preferably within the range from 150 to 300 nm.
Preferred dispersions have the following properties:
PTFE or PTFE copolymer powders useful for the invention preferably have bulk densities within the range from 300 to 1000 kg/m3, particularly preferably within the range from 400 to 600 kg/m3. The average particle size is preferably within the range from 20 to 1000 xcexcm, particularly preferably within the range from 250 to 700 xcexcm. The powders are preferably free flowing, especially powders having an average particle diameter of about 500 xcexcm and a bulk density of about 500 kg/m3. The PTFE or PTFE copolymer powders can be dispersed in a dispersion medium before use.
It may be advantageous in some instances that the solids content of the PTFE dispersion employed is reduced by adding water in order to obtain a desired concentration. A prediction of the necessary water amounts is not possible. The amount needs to be adapted in each single case (for example 2 to 30%, more specific 5 to 10% when using a 60% dispersion.
The PTFE or PTFE copolymer powders can also be used without being first dispersed in a dispersion medium. This has the advantage that no dispersion medium has to be removed. However, it is nonetheless preferable to add to the powders a surfactant in an amount of 1-15%, based on the PTFE weight. The surfactant can be added before, during or after the mixing of the components in step (a), but in any event before the heating [step (b)]. Surfactants used are preferably nonionic surfactants. Preferably they are compounds based on oxo alcohols or fatty alcohols having 10-18 carbon atoms, alkylphenols, fatty acids or fatty acid amides, which all contain polyethylene oxide radicals having 3-20 ethylene oxide units, or they are surfactants based on oleic acid alkoxylate, fatty alcohol alkoxylate, fatty acid alkoxylate or alkylphenol alkoxylate. Particular preference is given to using surfactants based on alkylphenols with polyethylene oxide radicals containing from 6 to 20 ethylene oxide units (e.g., Lutensol(copyright) AP6 from BASF).
Modified PTFE types may be employed as the PTFE. The modified PTFE contains small amounts of appropriate comonomers. Appropriate comonomers are e.g. hexafluorpropylene, perfluor(propylvinylether), ethylene, chlortrifluorethylene, venylidenfluoride. Preferably perfluorinated comonomers are employed. Modified PTFE powders may be obtained from Dyneon under the brand Hostaflon(copyright) TFM. They contain less than 1% of a comonomer.
PTFE copolymers may contain larger amounts of comonomers, for example 7 to 8 mol.-%. Among the preferred comonomers hexafluoropropylene (FEP) and perfluoro(propylvinylether) (PFA) the comonomers disclosed in U.S. Pat. No. 5,192,473 may be employed.
The weight ratio of PTFE or PTFE copolymer to finely divided inorganic material, without fiber forming material, is preferably within the range from 0.2 to 0.6, particularly preferably within the range from 0.25 to 0.5, especially within the range from 0.28 to 0.43.
The following is a description of the preferred embodiment of the invention:
The finely divided inorganic material and the fiber forming material are introduced into the Eirich mixer and briefly mixed through. The cylinder of the mixer is then set rotating, the rotor is switched on and then the PTFE or PTFE copolymer dispersion is added. It is possible to add the components in any desired order. Whatever the order of addition, the rotor should be on, to effect thorough mixing.
The rotor is then switched off or adjusted to an appropriate level, e.g. 450 Upm, and the mixing pan is allowed to rotate at low speeds of preferably not more than 100 rpm while the mixture is heated to the desired temperature. The temperature range for fiber formation depends on the material used. In general, the temperature is more than 70xc2x0 C., for example within the range from 80 to 200xc2x0 C. The water present in the dispersion is removed in this step, so that it should be carried out at temperatures below 100xc2x0 C. and reduced pressure. Reduced pressure can also be employed at higher temperatures in order that the removal of the water and, where appropriate, the dispersant may be speeded up.
The heating preferably takes from 0.25 to 2 hours. The heating time depends on the design and size of the mixer and on the type of heating and can also be more than 2 hours in the case of lower heating power. In the field, values of up to 6 hours are uncritical. Heating can be effected, for example, via wall heating or by introduction of high temperature steam (superheated steam).
Once the desired temperature is reached, fiber formation will generally be substantially complete. Mixing may continue at that temperature for a further 5-240 min.
The mixer contents are then allowed to cool down again. This is most simply done by allowing the contents to stand, i.e., without further mixing. However, during cooling, mixing may also be continued or a coolant such as cold air blown in or water blown in and subsequently evacuated for faster cooling.
Once the temperature is below 70xc2x0 C., preferably within the range from 20 to 60xc2x0 C., the rotor is switched on to comminute the clumped fiber material. The rotor is preferably set to a speed within the range from 300 to 2500 rpm. The mixing time is preferably within the range from 10 sec to 60 min. The mixing speed and the mixing time depend on the desired degree of comminution. In general, mixing times from 1 to 1.5 min are sufficient at a speed of 2500 rpm and from 1 to 5 min at a speed of 450 rpm.
Thereafter the free falling fiber material can be discharged in a simple manner.
The composite fibers obtained constitute a dry, free flowing, finely divided material. The fibers are fibrillike, anisotropic and of irregular morphology. The color depends on the inorganic material used and the PTFE polymer or copolymer. Each individual fiber can be branched or unbranched. Inorganic material is uniformly dispersed within the entire fiber and intimately mixed with the PTFE or PTFE copolymer as polymeric binder, so that it cannot be removed without destruction of the fiber. In addition, there is finely divided inorganic material on the surface of the fiber.
The composite fibers preparable or prepared according to the invention are useful for preparing diaphragms, especially chlor-alkali electrolysis diaphragms.
The invention also provides the process for producing diaphragms by
(A) preparing composite fibers by one of the aforementioned processes,
(B) introducing the composite fibers into a solution comprising water and a thickener for increasing the viscosity,
(C) suction filtering the mixture from (B) through a porous base to deposit the composite fibers on the porous base,
(D) drying the coated porous base from (C),
(E) thermally treating the diaphragm from (D) at from 90 to 390xc2x0 C.
The diaphragms can be prepared as described in EP-B 0 196317. The porous base used can be, for example, a cathode which is in the form of a grid and covered with a polyamide network.